1. Field of the Invention
The present invention relates to an improved process for the preparation of high molecular weight polyamides from cyano-containing monomers by polymerization in a manner known per se to give polyamides having a low molecular weight and subsequent thermal aftertreatment to give high molecular weight polyamides.
The present invention furthermore relates to high molecular weight polyamides, obtainable from aminonitriles and/or a mixture of diamines with dinitriles, the use of the novel polyamides and moldings obtainable therefrom.
2. Description of the Related Art
U.S. Pat. No. 2,245,129 describes the preparation of linear polyamides. Here, an aminonitrile or an equimolar mixture of dinitriles and diamines is converted in a first step, initially to low molecular weight polyamides. In a subsequent second step, the low molecular weight polyamides are subjected to thermal postcondensation by removing ammonia and water at atmospheric pressure or reduced pressure. The disadvantage of this process is, on the one hand, the low polymerization rate in the postcondensation, which is evident from only a slight increase in the relative viscosity of the end product compared with the relative viscosity of the low molecular weight polyamide in the case of long postcondensation times. However, long postcondensation times increase the probability of damage to the polymer and lead to higher energy consumption and to low space-time yields. On the other hand, the end products have a ratio of terminal amino groups to terminal carboxyl groups (TAG/TCG) substantially greater than 1, which adversely affects the melt stability (constancy of molecular weight) during processing.
DE-A 35 34 817 describes the stepwise preparation of linear polyamides from omega-aminoalkanenitriles and excess water in the presence of catalytic compounds. Spinnable polyamides are brought to the desired higher degrees of polymerization in the melt in an inert gas atmosphere, such as nitrogen, or at reduced pressure. The polyamide thus obtained is then granulated, freed from monomers and oligomers by extraction and then dried. Here too, the achievable molecular weights are low, and the TAG/TCG ratio is likewise substantially greater than 1.
EP-A 479 306 describes the preparation of polyamides starting from omega-aminonitriles in the presence of water and with continuous addition of water, both steam and ammonia being removed from the reaction mixture at above 240.degree. C. After the addition of a certain amount of water, the water feed is stopped and the polymer obtained is subjected to final condensation at from 240.degree. to 310.degree. C. until the desired molecular weight is obtained. It is not possible to obtain higher molecular weights owing to the autogenous steam pressure, which shifts the polycondensation equilibrium to low molecular weights. Furthermore, the reaction rate and hence the space-time yield are not sufficiently high.
EP-A 129 195 describes the preparation of high molecular weight polyamides. In the process according to EP-A 129 195, a low molecular weight prepolymer is first prepared from a mixture consisting of diamines and dicarboxylic acids and is then subjected to solid-phase postcondensation with superheated steam until the desired molecular weight is obtained.